Integrated sedimentological and ichnological study of the Coniacian sedimentation in North Sudetic Basin, SW Poland

Lithofacies and ichnological features of the Coniacian deposits of the upper part of Żerkowice Member and lowest part of the overlying Czerna Formation in southeastern part of the North Sudetic Synclinorium are described and their sedimentary palaeoenvironment is interpreted. The study confirms a shallow-marine to paralic/paludine palaeoenvironment. Sedimentation of the Żerkowice Member occurred in an upper shoreface environment dominated by waves, tidal currents and wave-generated alongshore currents, with an episodic encroachment of fore shore zone and shoal-water deltas. The interpretation is supported by a high-diversity assemblage of trace fossils with 21 ichnogenera, representing a stressed expression of the Skolithos Ichnofacies dominated by Ophiomorpha nodosa and a proximal expression of the Cruziana Ichnofacies with Thalassinoides and rare specimens of diverse other ichnotaxa. Sedimentation of the Czerna Formation commenced after a stasis, with at least a local hiatus caused by emergence, and proceeded in a laterally and vertically more varied environment, with transgressive coastal lagoons evolving into freshwater lakes and marshes and with a repetitive regressive intrusion of shoreface and shoal-water deltas. The emergence of the area is recorded by coal-bearing deposits with plant-root traces. Local occurrence of the Teredolites Ichnofacies in coal (peat) deposits above the base of the Czerna Formation indicates renewed marine flooding. Continuation of the latter is locally evidenced by a trace-fossil assemblage with 17 ichnogenera, representing proximal expression of the Cruziana Ichnofacies followed by distal expression of the Skolithos Ichnofacies in the overlying transgressive-regressive cyclothems. The palaeoenvironmental changes recorded by the sedimentary succession indicate bathymetric fluctuations and imply considerable shoreline shifts and palaeogeographic changes in the basin. These changes are interpreted as a combined signal of 2nd- and 3rd-order eustatic cycles, modified and partly obliterated by the effects of intrabasinal tectonic forcing and by palaeogeographically controlled variation in sediment supply.